feat(backend): implement graceful shutdown for container stop

Graceful shutdown ensures in-flight operations complete before process exits:
- Lifespan shutdown handler drains pending tasks with 25s timeout
- Scheduler stops accepting new jobs immediately
- HTTP session, external logging, scheduler lock, DB conn closed cleanly
- 25s Python timeout leaves 5s margin before Docker's 30s SIGKILL

Files changed:
- backend/app/main.py: enhanced _lifespan shutdown with task drain
- Docker/Dockerfile.backend: documented signal handling in header
- Docker/docker-compose.yml: added stop_grace_period: 30s
- Docker/compose.prod.yml: added stop_grace_period: 30s
- Docs/Deployment.md: new Graceful Shutdown section with sequence table
- Docs/TROUBLESHOOTING.md: new Graceful Shutdown Issues section

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/models/auth.py

@@ -7,6 +7,7 @@ from pydantic import Field
 
 from app.models.response import BanGuiBaseModel
 
+
 class LoginRequest(BanGuiBaseModel):
     """Payload for ``POST /api/auth/login``."""
 
@@ -35,6 +36,12 @@ class LogoutResponse(BanGuiBaseModel):
 
     message: str = Field(default="Logged out successfully.")
 
+class SessionValidResponse(BanGuiBaseModel):
+    """Response for ``GET /api/auth/session`` confirming session validity."""
+
+    valid: bool = Field(default=True, description="Whether the session is valid and active.")
+
+
 class Session(BanGuiBaseModel):
     """Internal domain model representing a persisted session record."""